An isotope dependent study of acetone in its lowest excited triplet state

Abstract

The lowest excited triplet state T ₁ of acetone-h₆ and acetone-d₆ was investigated with a pulsed dye laser equipped ODMR spectrometer. The results obtained with this prototypic aliphatic carbonyl compound are discussed mainly with reference to a recent ab initio calculation on formaldehyde. Acetone is found to be bent in T ₁ and the out-of-plane distortion angle is estimated to be ≈ 38°, similar to the formaldehyde value. In view of the fact that T ₁ is of nπ* nature, the observed zero-field splitting (ZFS) is surprisingly small. Both the spin-spin and the spin-orbit (SO) contribution to the ZFS are evaluated and the pertinent axis systems, which do not coincide owing to the non-planar geometry, are specified. The SO tensor contribution is calculated from a correlation between the deuterium effects on the ZFS parameters and the population rates. The sub-level selective kinetics of the acetone T ₁ is largely determined by the mixing of the x- and z-level characteristics owing to magnetic axis rotation caused by the excited state out-of-plane distortion. Considerable deuterium effects are observed on the kinetic data and on the microwave transition frequencies. In all cases the spinspecific isotope effects (due to the promoting modes) and the global effects (due to the Franck-Condon factors) are specified. For the population rates and the SO contribution to ZFS, the inverse global isotope effect (deuterium factor >1) was found for the first time. Based on the isotope dependence of the rates, the mechanisms of (vibrationally induced) SO coupling in acetone are discussed. It is concluded that non-adiabatic contributions have to be taken into account for the smallest population rate only, but that otherwise the adiabatic SO coupling mechanisms by far dominates in the acetone photophysics.